Modulated carrier wave transmitter



Feb. 4, 1941. N. H. cLouGl-l Erm.

MODULATED CARRIER WAVE TRANSMITTER FiledDec. 16, 1938 2 Sheets-Sheet l Nil Feb- 4, 1941- N. H. cLoUGH Erm. 2,230,346

I MODULATED CARRIER WAVE TRANSMITTER Filed Dec. 16, 1938 2 Sheets-Sheet 2 |F+ H d .HIE a mb 555mm w u. au Sq l. |N\Nm svp. n mi n M, mex. N mu TE R a A NNW@ m .mx i wnzmkoq =V. E .M W M. u wztbuo mwN -H QG N 4 uo mugs@ wm E Hs. N.Q.T\. 1+ w m Q d Smo E m w .qu s 5f QT mtum Patented Feb. 4, 1941 UNITED STATES MODULATED CARRIER WAVE TRANSMITTER Newsome Henry Clough, Brentwood, and Ernest Green, Chelmsford, England, asslgnors to Radio Corporation of America, a corporation of Dela- Ware Application December 16, 1938,' Serial No. 246,045

In Great Britain January 1, 1938 This invention relates to modulated carrier 'wave transmitters oi the floating carrier type, i. e., 'of the type wherein the carrier level is automatically controlled by and in dependence upon the level of the modulating signals so that the carrier level is increased when strong modulating signals are present and vice versa.

It is usual in iioating carrier type transmitters in which the carrier power ls a function of 10 the rectiiled modulation envelope, to magnify the modulating potentials (which may be taken from any pair of points in the cascade speech amplifier) to rectify the magnified power and to pass the resultant direct current after smoothing and e direct current ampliiication, through a resistance in series with a bias source to the main modulating stage, voltages across the resistance, and thus the total bias of the main modulator, varying with the rectified modulation envelope, 20 Usually the modulation power isconverted to bi-phase by means of an iron cored transformer and is rectified by means of a full wave rectier. In order to maintain a, rectilinear performance for varying power and frequency such a system has to be so operated that the maximum current passed through the iron cored transformer produces only a comparatively weak flux in the core. Thus rectiiication is effected at lower power and the rectified output has to be amplified.

8 Buch a method has the disadvantage that it is expensive on account of the tube amplifiers, and that it is difficult to arrange the direct current arnpliiier so that its output can be applied to two points (both of which float with respect to earth- 85 or to a point in the plate supply source for the transmitter tubes) in the modulator grid network.

These difficulties are met, in arrangements in accordance with the invention set forth in the 4o specification accompanying co-pending application Serial No. 168,167, iiled October 9, 193'?, by eliminatlngthe inon cored transformer and effecting rectification by means of a therrnionlc device operating as an anode bend rectifier. Ac- 45 cording to the said invention in the said copending specification a series modulated carrier wave transmitter of the iloating carrier type, wherein iioating carrier action is obtained by varying under the control of modulating signals 60 the bias upon a modulator tube stage, is characterised in that said variation control is eiected by. a rectifier arrangement which includes an anode bend thermionic tube rectifier and which rectiiles the modulating signals to provide a co'n- I6 trolling output, the rectifying means in 4said arfixed component such that in the absence of controlling output from the rectifier arrangement (i. e., in the absence of modulation) the carrier is at predetermined level. Arrangements may also be made for providing different operation and 10 release times, the usual requirement being a quick operating and a slow release time.

In general, the fixed bias lof the modulating stage is such that in the quiescent condition, i. e., in the absence of modulation, the carrier is at a previously selected value and the anode bend rectiiler is so blassed that its anode current is zero or is well down the bend.

The circuits are so arranged that when modulation potentials are applied to the rectifier, the rectifier becomes conductive on each positive half cycle andthe resultant increase of rectifier current increases the carrier level.

In the various circuits shown and particularly described in the specification above mentioned the. uni-directional bias potential which adjusts the carrier level automatically is a negative bias potential which increases with increase in modulation level. In some transmitters, however, it 'is required, in order that the desired carrier adjustment shall be effected, that the adjusting bias potential shall decrease with increase in modulation level. One form of transmitter presenting this requirement is a series modulation transmitter wherein the carrier level is directly controlled by the bias of the main modulator tubes; another is a transmitter wherein a pentode tube is used as the modulating tube (receiving a. carrier input on one grid and a modulating input on another electrode) and the can'ier is con- 40 trolled by bias applied to the so-called suppressor grid. The present invention seeks to provide circuit arrangements which will operate in the manner required for such transmitters as this and at the same time present the advantages of the arrangements described in the above mentioned specification.

According to this invention a carrier wave transmitter of the floating carrier type and which is such that, for floating carrier control, a bias potential which decreases with increase in modulation level is required, is characterised in that said bias potential includes the voltage drop across a direct-current permeable impedance which is in the anode-cathode circuit of an anode bend embodiments. The embodiments of Figs. 1, 2, 3

and 4 each involve modulators wherein the carrier level is controlled by controlling the bias on a grid electrode of a tube in accordance with a potential derived by rectifying the modulating potentials. Figs. 1, 2 and 3 differ mainly in the means for obtaining and applying the controlling potential. The modulators of Figs. 1, 2 and 3 are of the series-modulation type whereas in the embodiment of Fig, 4 a single tube ampliiies and modulates the. carrier.

Referring to Figure 1 which shows one way of carrying out the invention the anode circuit of any convenient tube VI in the modulation chain of the transmitter to which the invention is to be applied-usually the sub-modulator tube-has its anode connected to its cathode through a series circuit comprising, in the order stated, a first condenser C2, a potentiometer resistance R2 and a bias source comprised of two portions B2 and Bi in series, the cathode being earthed. An anode bend rectifier V3, for example a triode as shown, has its control grid connected to a preferably adjustable tap on the potentiometer resistance R2 and its cathode is connected through a second resistance R3 to a tap between the-por tions B2, Bl. This tap which is preferably ad- ,instable along the source B2, BI is so chosen that normally, i. e. in the absence of modulating potentials, the rectifier is cut-off. The rectifier, if of the filament type, should have its cathode heated from an insulated heating -voltage source or a rectiiier of the indirectly heated cathode type may be employed. The anode of the rectifier is connected to its cathode through a second condenser C3 and is also connected to the cathode of the tube VI (i. e. earth) and to the cathode of a further tube V2 in the transmitter-namely that to whose grid the carrier level controlling bias potential is applied. The grid of the tube V2 is connected to the cathode of the rectifier V3 through a third resistance RI. 'I'he tube V2 might, for example, be the main modulating tube in a series modulation systemin which case the grid thereof mentioned would be its control grid, or it might, to give a further example, be a pentode tube supplying power to an oscillating circuit and the grid thereof mentioned be'its suppressor grid, as shown in Figure 4. Whatever the nature of the further tube, V2, since, in the absence of modulating potentials, the rectifier is at cut off and there is no potential drop across the resistance R3, the bias on the grid of said further tube V2 will be at a maximum. Upon the appearance of modulation input, however, the rectifier V3 conducts on the positive half cycles and accordingly the voltage on the grid of the tube V2 is reduced, in proportion to the modulation swing, by reason of the opposing voltage set up across the resistance R3. The resistance R3 and the condenser C3 are suitably proportioned to give required suitable growth" and decay time factors. Where required, A. C. modulation can also be applied via a coupling condenser CI, from the anode of the tube VI to the grid of the tube V2.

In the modification shown in Figure 2 the cathode of the rectifier V3 is connected to earth and to the cathodes of the other two tubes VI and V2 through the second resistance R3 in series with a source of bias potential BI, and the anode of the tube Vi (the submodulator tube) is connected to the cathode of the rectifier V3 (instead of to l its own cathode) through a series circuit comprising, in the order named, the first condenser C2, the potentiometer resistance R2 and a bias source B2. In other respects the circuit is as in Figure 1.

In a further modification shown in Figure 3 the junction point of the anode of the rectifier V3 with the cathode of the tube V2 is connected through a resistance R3 to the positive terminal of a floating bias source B'2 an adjustable tap on which is connected to the cathode of the rectifier V3. A second adjustable tap on the no ating source B'2 is connected through a resistance RI to the grid of the further tube V2, said grid being capacitatively coupled by condenser Ci to the anode of the first tube VI. The other connections are as in Figure 1, with this arrangement the fioating source B'2 provides anode potential for the rectifier V3 and standing bias for the further tube V2, cut off bias for the rectier being obtained by reason of difference of potential between the source B2 which is in series with the potentiometer resistance R2 and the portion of the floating bias source B'2 which is on the positive side of the tap leading to the cathode of the rectifier V3. On the application of modulating potential to the rectifier grid the rectifier passes current through the resistance R3 in series with the floating source B'2 across which there accordingly occurs a potential drop which opposes the standing bias on the further tube V2 and reduces the total bias thereon.

In Fig. 4 the tube V2 is replaced by a pentode V2 having a high frequency input circuit LI C4 connected to one grid and a high frequency output circuit C5 L2 connected to the anode.

The modulated stage V3 in Figs. l to 3 inclusive may be as shown therein. In this circuit C3 bypasses high frequency but is of high resistance to audio frequency. The operation of the modulated stage is otherwise obvious.

What is claimed is:

1. In a wave modulation system, an electron discharge device having input and output electrodes, means for impressing wave energy to be modulated on said input electrodes, means for deriving modulated wave energy from said output electrodes, means for impressing modulating potentials on an electrode of said device, and 55 means for impressing a potential on an electrode of said device the value of which depends on the mean amplitude of the modulating potentials comprising, an amplifier tube having input electrodes including a cathode excited by said modulating potentials and having an anode, a rectifier tube having an anode, a cathode and a control grid, a circuit including a resistance and a source of direct current potential connected between the control grid and cathode of said rectiiier tube for biasing said rectifier tube substantially to cut-ofi', a resistance .and a source of potential connected between the anode and cathode of said rectifier tube, means connecting said last named resistance between the cathode of said 70 rectifier tube and the cathode of said amplifier tube, means for impressing amplified modulating potentials from the anode of said amplifier tube on the control grid of said rectifier tube whereby a drop is produced in said resistance between u the anode and cathode of said rectifier tube, and means for impressing a potential on an electrode of said electron discharge device which varies in accordance with the potential drop produced in said last named resistance by rectification of said modulating potentials.

2. In a wave modulation system, an electron discharge device having input and output electrodes, means for impressing wave energy to be modulated on said input electrodes, means for deriving modulated wave energy from said output electrodes, means for impressing modulating potentials on electrodes of said device, and means for impressing a. potential on an electrode of said device the value of which depends on the mean amplitude ofthe modulating potentials comprising, an amplifier tube having input electrodes including a cathode excited by said modulating potentials and having an anode, a rectifier tube having an anode, a cathode and a control grid, a circuit including a resistance and a source of direct-current potential connected between the control grid and cathode of said rectifier tube for biasing said rectifier tube substantially to cut-off, a resistance and a source of potential connected between the anode and cathode of said rectifier tube, means connecting the cathode of said rectifier tube to the cathode of said amplifier tube, means for impressing amplified modulating potentials from the anode of said .amplifier tube on the control grid of said rectifier tube whereby a drop of potential is produced in said resistance between the anode and cathode of said rectifier tube, and a connection between said last named resistance and electrodes of said device whereby the impedance between the anode and cathode of said device is controlled in part by the value of the potential drop in said resistance.

3. In a wave modulation system, an electron discharge device having a cathode, an anode and a plurality of control grid electrodes, means for impressing wave energy to be modulated on a -control grid electrode and said cathode, means for impressing modulating potentials on a con-l trol grid electrode and said cathode, means for deriving modulatedwave energy from the anode and cathode of said device, and means for impressing a potential on a control grid electrode of said device the value of which depends on the mean amplitude of the modulating potentials comprising, an amplier tube having input electrodes including a cathode/excited by said modulating potentials and having an anode, a rectifier tube having .an anode, a cathode and a control grid, a circuit including a, resistance and a source of direct-current potential connected between the control grid and cathode of said rectiiier tube for biasing said rectifier tube substantially to cut-01T, a resistance .and a source of potential connected between the anode and cathode of said rectifier tube, means connecting the cathode of said rectifier tube to the cathode of said amplifier tube and to the cathode of said electron discharge device, a connection between said last named resistance and a control grid of said electron discharge device, and means for impressing amplified modulating potentials from the anode of said amplifier tube on the control grid of said rectifier whereby the cut-off bias of said rectiiier is overcome and current flows in said resistance connected between the anode and cathode of said rectifier tube to produce therein a potential depending upon the amplitude of the modulating potentials impressed on the control grid of said rectifier tube.

NEWSOME HENRY CLOUGH. ERNEST GREEN. 

